The present invention relates to an image forming apparatus capable of controlling in such a way that uniform fixing can be performed at a high speed under stable conditions in conformity to the quality of the paper, without wrinkle or uneven gloss, even if the thickness and gloss thereof differ.
In the thermal fixing device of a color image forming apparatus, the heating roll engaged with a unfixed toner image surface must be composed of a material having an elastic layer, unlike the monochromatic image forming apparatus, as described on page 70 of the proceedings read at the 42nd seminar of the Japan Society of Electrophotography in 1996. It comprises a cylindrical cored bar of such a metal as aluminum or stainless steel coated with a heat resistant elastic layer of silicone rubber, etc. to a thickness of about 0.5 to 3 mm. In some cases, it has a thickness of 15 to 17 μm by coating a highly heat-resistant fluorine resin such as PFA and PTFE or by tube coating, in order to improve the durability and mold releasing property on the surface. As a heat source, a halogen lamp is fixed in the space inside the metallic cored bar of a heating roll to control current application, or an insulation thin film is provided inside the metallic cored bar and a resistance heating element is provided further inside to control current application. Such a configuration is known so far. In the configuration of the normal heating roll fixing device, a heating roll is installed on the upper side, and a pressure roller is installed on the lower side in many cases. When the heating roll (hereinafter referred to as “heating roll” or “upper roll”) and pressure roll (hereinafter referred to as “pressure roll” or “lower roll”) engaged therewith have been brought into pressure contact with each other, a nip is convex on the top as the elastic layer of the upper roller is thicker, as shown in the schematic diagram of FIG. 11(a), and tends to move away from the upper roll in the tangential direction of the lower roll. This is advantageous for self-stripping performance that allows separation without using any means for stripping paper forcibly from the upper roll as a heating roll. Further, nip width can be secured by the small-diameter roll as the elastic layer of the upper roll is made thicker. The above-mentioned description is disclosed in the Japanese Publication Tokkaisho No. 55-17108 and on page 38 of Fuji Xerox Technical Report No. 9 (1994). However, the thermal conductivity of silicone rubber or the like is smaller than that of metal. Accordingly, as the elastic layer of the upper roll is made thicker, thermal conduction from the heat source to the roll surface becomes poorer; with the result that thermal deterioration is caused by increased warming-up time and temperature rise at the metallic cored rod. Further, deterioration of heat conduction from the heating plate to the roll surface causes the surface temperature to be reduced when paper is passed through, and makes it difficult to increase speed. If the heating roll rubber is made thinner in an attempt to improve thermal conduction from metallic cored rod for higher speed, then the fixing nip becomes flat or convex on the bottom, as shown in FIG. 11(b). It winds around the upper roll as a heating roll, and cannot easily be removed. If the nip becomes convex on the bottom, self-stripping will be difficult. This makes it necessary to increase the diameter of the heating roll considerably when a required nip width is to be secured. Self-stripping is not easy, when a pressure belt is adopted as a pressure member for forming a fixing nip to secure nip width in order to ensure a high speed, as disclosed in the Japanese Publication Tokkaihei No. 5-150679 (Fuji Xerox Co., Ltd.).
Methods for solving this problem is disclosed in Japanese Publications Tokkaihei No. 8-314323, No. 10-10919, No. 10-97150, No. 11-721, No. 11-24465 and No. 11-38802. As disclosed, an external heating roll without elastic layer is brought in rotary contact with the surface of the heating roller, whereby ensuring an efficient supply of heat to the heating roll. Alternatively, a heating source is provided on the pressure roll in order to ensure that not much heat will be removed from the heating roll by the pressure roll.
However, even if many heat sources are provided, the maximum power consumption is increased if power is supplied at one time. When a heat source is provided on the pressure roll side, the pressure roll temperature must be kept low if the second surface is copied in the duplex copying mode. Otherwise, this will increase the difference of the image glossiness between the front and back. When coated paper is copied under highly humid conditions, pressure roll temperature must be kept low. Otherwise, a blister may occur, as disclosed in the Japanese Publication Tokkaihei No. 11-194647. It is difficult to switch over the preset temperature earlier using the heat source located inside the pressure roll. If the pressure roll is made into a hard roll, the set temperature can be easily switched over earlier, but the image on the first surface will be deteriorated in the duplex copying mode. This will raise a problem.
In recent years, as disclosed in the Japanese Publication Tokkai No. 2000-347454, wax of a low melting point is dispersed and mixed in toner, and the surface of the heating roll is coated with fluorine resin, whereby reduced cost, easy writing on the hardcopy, easy bonding of tape, and greater transparency of the transparent sheet are ensured, without having to use the means of coating a mold releasing agent to the heating roll, which is the conventional technique. To achieve these objects, it is important to secure a self-stripping performance, as mentioned above, and to allow the fixing nip to be formed convex on the top, as viewed from the side. However, as described in the Konica Technical Report (1997), if the upper convex shape of the nip is excessive, then wrinkles will occur at the overlapped portion of paper for example, in envelopes. To solve this problem, it has been common practice to select the conditions permitting compatibility between wrinkles of the envelope and self-stripping performance, as disclosed in the Japanese Publications Tokkaihei No. 05-265344 and Tokkai No. 2000-321913. But it has been necessary to apply several milligrams of silicone oil per page, as described in Japanese Publication Tokkai No. 2000-321913. Accordingly, in the oil-less fixing mode without mold releasing agent being applied to ensure self-stripping performance while preventing the wrinkles of an envelope, it is necessary to give effects other than nip profile.
At present, it is common practice to prepare several modes of changing a preset temperature, process speed and other factors of the heating roll and pressure roll for a great variety of paper types different in thickness, gloss and surface properties. According to the limited information of the developer, multiple modes are determined under the name of so called paper mode or gloss mode. For limited paper types, a corresponding mode is specified so that a desired fixing property and glossiness can be obtained. For other paper types, the user selects the mode manually as appropriate. Repeating trials and errors, the user selects a preferred mode. Under these circumstances, it cannot be said that the user requirements are met in a limited number of modes for a variety of paper types. Further, if the user feeds the paper whose stiffness is lower than assumed by the developer, the aforementioned self-stripping performance cannot be secured. So paper will wind round the heating roller and the machine will get faulty. This failure must be repaired by a service engineer. This problem can be solved when a separating member that forcibly removes the paper is engaged with the surface of the heating roll. However, the portion of the separating member engaged with the heating roll is likely to deteriorate to cause a defective image to be produced. Further, if too much heat is supplied to the coated paper where different materials are coated on the surface of normal paper substrate, especially in the paper such as art paper and coated paper with a great amount of coating material applied thereon, then steam generated from inside the paper substrate is hindered by the coated layer and cannot get out of paper, with the result that a blister specific to coated paper may be caused in some cases. For plain paper, it is preferred to change the fixing conditions for control. So far, change of fixing conditions has been made by manual selection of the aforementioned mode. Further, for the same paper type, fixing conditions have been changed according to the moisture content of paper and the type of copying mode (simplex or duplex copying). However, fixing conditions have not been changed according to the combination with paper qualities such as thickness, gloss or stiffness as inherent properties of paper.